Prognostic Meteorological Discussion
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FXAK02 KWNH 281851
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ALASKA EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
251 PM EDT SUN MAY 28 2017

VALID 12Z THU JUN 01 2017 - 12Z MON JUN 05 2017

BASED ON THE ARRAY OF LATEST AND RECENT GUIDANCE THE MEAN RIDGE
EXPECTED TO EXTEND FROM CANADA ACROSS MAINLAND ALASKA CONTINUES TO
BE A MORE CONFIDENT ASPECT OF THE FCST, IN CONTRAST TO THE AREA OF
RELATIVELY LOW HGTS ALOFT FROM NEAR THE ALEUTIANS INTO THE NERN
PACIFIC WHERE THERE IS STILL SOMEWHAT GREATER SPREAD AND
RUN-RUN/DAY-DAY VARIABILITY FOR SPECIFICS.

A KEY UNCERTAINTY YET TO BE RESOLVED INVOLVES THE ULTIMATE
EVOLUTION/PATH OF CLOSED LOW ENERGY NEAR THE ALEUTIANS AT THE
START OF THE PERIOD 12Z THU.  AS OF THE 00Z-06Z CYCLES THERE WAS A
FULL RANGE OF POSSIBILITIES, FROM THE PAST COUPLE ECMWF RUNS
HOLDING THE ENTIRE UPR LOW NEAR THE ALEUTIANS TO THE 00Z UKMET/CMC
WHICH HELD SOME BACK AND PROGRESSED THE REST, AND THEN THE 00Z-06Z
GFS/00Z NAVGEM WHICH BROUGHT THE WHOLE FEATURE EWD.  SO DOES THE
12Z GFS.  IN SPITE OF THE SPREAD THAT EXISTS EVEN AMONG INDIVIDUAL
GEFS/ECMWF/CMC ENSEMBLE MEMBERS FOR THIS ASPECT OF THE FCST, THERE
IS A NOTABLE TREND IN THE MEANS OVER THE PAST 24 HRS TOWARD
GREATER/EARLIER EMPHASIS ON ALEUTIANS ENERGY-- FROM SOME
COMBINATION OF THE INITIAL CLOSED LOW AND/OR ENERGY UPSTREAM--
RELATIVE TO THE FEATURE OVER THE NERN PAC.

BY THE LATTER HALF OF THE PERIOD THE 00Z ECMWF/CMC ENDED UP
CLUSTERING DECENTLY WITH THE ENSEMBLE MEANS FOR THE POTENTIAL
SYSTEM NEAR THE ERN ALEUTIANS/ALASKA PENINSULA.  THUS SOME
WEIGHTING OF THOSE RUNS COULD BE INCLUDED TO ENHANCE THE MEANS A
BIT FROM LATE DAY 6 SAT ONWARD, BUT AT A LOW ENOUGH LEVEL NOT TO
HAVE AN ADVERSE EFFECT ON THE MORE AGREEABLE/CONSISTENT ENSEMBLE
MEAN DEPICTION OF THE MAINLAND RIDGE ALOFT OR COMMIT TOO STRONGLY
TOWARD ANY DETERMINISTIC SOLN FOR UNCERTAIN LEADING SHRTWV ENERGY
THAT MAY TRACK OVER OR S OF THE PANHANDLE.  REGARDING MAINLAND
FLOW ALOFT THERE ARE A VARIETY OF SOLNS BUT THE 00Z AND 12Z GFS
RUNS ARE MORE ON THEIR OWN THOUGH, WITH THE 00Z RUN BRINGING MORE
PROGRESSIVE ARCTIC FLOW ACROSS NRN AREAS SAT ONWARD AND THE 12Z
RUN BRINGING AN UPR LOW MUCH CLOSER TO THE NWRN MAINLAND THAN
OTHER SOLNS.

AT LEAST GIVEN THE TIME FRAME INVOLVED THERE IS DECENT AGREEMENT
THAT A SYSTEM SHOULD EMERGE OVER THE WRN PAC AND PSBLY BEGIN
AFFECTING THE WRN ALEUTIANS BY DAY 8 MON.  THE 00Z ECMWF/CMC HAD
TIMING SIMILAR TO THE MEANS BUT WITH A SOMEWHAT FARTHER SWD TRACK,
AGAIN FAVORING JUST ENOUGH MODEL INPUT TO ENHANCE DETAILS WITH THE
DOWNSTREAM SYSTEM WHILE OTHERWISE MAINTAINING THE ENSEMBLE MEAN
THEME FOR THIS TRAILING WRN FEATURE.

BASED ON THE ABOVE CONSIDERATIONS, OVERALL PREFERRED TO START WITH
A BASE OF ABOUT 2/3 ECMWF MEAN AND 1/3 06Z GEFS MEAN TO REFLECT
RECENT TRENDS WHILE NOT COMMITTING TOO STRONGLY TO ANY SPECIFIC
MODEL RUN.  GIVEN THE UPR LVL EVOLUTION IN THE MEANS AND
COORDINATION WITH WFO AFC, THIS BLEND INCLUDED SOME 00Z ECMWF
INPUT THROUGH THE PERIOD WHILE AFOREMENTIONED CLUSTERING FOR THE
ERN ALEUTIANS SYSTEM ALLOWED FOR A LITTLE 00Z CMC INPUT AS WELL
LATE SAT ONWARD.  NEW 12Z GUIDANCE AS A WHOLE DOES NOT PROVIDE
MUCH CLARITY WHERE GREATEST UNCERTAINTIES EXIST, THOUGH CONSENSUS
REMAINS IN FAVOR OF LEANING AWAY FROM 00Z/12Z GFS RUNS FOR FLOW
ALOFT OVER THE MAINLAND.

RAUSCH

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